1. Field of the Invention
The present invention relates to a method for manufacturing an array of diamond cylinders each having a dent in its end portion.
2. Description of Related Art
A diamond cylinder structural body, especially a diamond cylinder structural body provided with electrical conductivity by doping is used for an electron emission source for a display, a gas sensor, an electrode material, and the like. For this purpose, it is usually important to form a fine and regular structure for improving performances. As an effective method for manufacturing such a cylinder-shaped diamond, there is known a method wherein a structure having a regular construction is previously prepared and vapor deposited diamond is formed thereon by using this structure as a mold and then the mold is dissolved out to form protrusion-like diamond with a uniform shape [K. Okada et al., Applied Physics Letters, vol.64, p.2742 (1994)]. The regular structure used as the mold is provided by working silicon or the like through a usual lithography technique.
For the applications of the cylinder structural body, it is important matters to minimize a diameter of each cylinder and increase a ratio of cylinder length/cylinder diameter or an aspect ratio. For example, the minimization of the diameter of the cylinder becomes an important matter in order to more efficiently concentrate electrical field into an end of a needle-like structure in an electron emission source.
For this purpose, the inventors have proposed a method wherein an anodized porous alumina, which is known to have a regular porous structure, is used as a mold and diamond is grown thereon by a CVD process and thereafter the anodized porous structure is dissolved out to obtain a cylinder-shaped diamond with a high aspect ratio [Masuda et al., The Electrochemical Society of Japan, 1998 Autumn Time Annual Proceedings, 1c18(1998)]. According to this method, it is possible to produce an array of fine cylinders at a low cost and a high throughput. In addition, a mold structure (anodized porous alumina) having a high ratio of pore depth/opening diameter (aspect ratio), which has never been formed in the usual lithography, is obtained, so that an aspect ratio of a cylinder structural body formed is made higher. At the same time, the inventors have disclosed a technique wherein fine diamond particles as a growth nucleus are uniformly applied onto inner walls of pores in the anodized porous alumina to obtain a cylindrical diamond-like carbon having a hollow portion in its top face [Masuda et al., Proceedings of the aforementioned lecture]. In this technique, a cylindrical structure is formed by uniformly distributing fine diamond particles as a growth nucleus on the wall faces of the pores in the anodized porous alumina and at the same time starting the growth of diamond-like carbon from the whole wall face.
When the array of diamond cylinders is formed by using the anodized porous alumina as a mold as mentioned above, the top portion of each of the cylinders formed has a flat shape. When such a cylinder array is used in an electron emitter, there is a problem that the concentration of electrical field is difficult. And also, if it is intended to apply such an array to an electrode or the like, it is difficult to carry other substances (catalytic metal, enzyme, etc.) on the top portion of the cylinder.
On the other hand, the cylinder having a hollow portion formed by uniformly applying the growth nucleuses on the wall face to conduct the CVD process consists of diamond-like carbon having a crystal form different from that of diamond, so that there is a problem in the use for the electron emitter or the electrode.
It is, therefore, an object of the present invention to provide a method for forming a dent or a hollow portion opening at a top face of a diamond cylinder.
The inventors have found that when a cylinder array of diamond is subjected to a plasma etching from the top face side of the cylinder, a dent opening toward the top side can selectively be formed in each cylinder. In other words, it has been found that only a central portion in the top face of each of the diamond cylinders is selectively etched, but a peripheral edge portion of each cylinder remains as a cylindrical or annular protrusion without being etched. In this case, a mask is not used. Such a selective etching phenomenon is not known up to this day.
Therefore, it is possible to obtain a cylinder-shaped diamond having a hollow structure in which only the central portion is selectively etched by adjusting an etching time and also it is possible to adjust a depth of a dent or a ratio of depth to width in the dent.
The thus obtained cylinder array having dents therein can be expected to have a high efficiency as an electron emitter. And also, it is possible to carry a given substance on the dent in the top portion of the cylinder. Therefore, the present invention can be said to be a basic manufacturing method in the field of diamond electronic parts, which will be expected to develop in the future.
In the present invention, the number and shape of cylinders included in the cylinder array are not critical. However, the width (diameter) of the cylinder is preferable to be 100-300 nm, and the height thereof is preferable to be 1-10 xcexcm. Furthermore, the ratio of height to diameter in the cylinder (aspect ratio) is preferable to be not less than 3.
Particularly, the cylinder can be formed in accordance with the form of the pore in the anodized porous alumina by using the anodized porous alumina as a mold and forming diamond in each pore of the anodized porous alumina through a vapor phase growing process before plasma etching.
The anodized alumina is a porous oxide film formed by anodizing aluminum in a proper acidic electrolyte, wherein fine straight pores having a high aspect ratio are arranged at approximately equal intervals. The pore diameter, pore interval, and pore length can be controlled by adjusting anodization conditions. The anodized alumina can be rendered into a self-supporting film by selectively dissolving out an aluminum matrix after the anodization. Furthermore, the fine pores can be made to through-holes by dissolving out a bottom portion of the film through an etching treatment.
A cylinder-shaped diamond having a high aspect ration can be obtained by using the anodized alumina as a mold in the formation of diamond film and growing diamond through the CVD process and thereafter dissolving out the anodized alumina.